In recent years there has been an intensive search for new high strength metallic materials having low relative weight, good ductility, workability, formability, toughness, fatigue strength and corrosion resistance. These new materials are destined for aerospace, automotive, electronic and other industrial applications.
The use of powder metallurgy techniques and, more particularly, mechanical alloying technology has been keenly pursued in order to obtain these improved properties. Additionally, powder metallurgy generally offers a way to produce homogeneous materials, to control chemical composition and to incorporate dispersion strengthening materials into the alloy. Also, difficult to handle alloying materials can be more easily introduced into the alloy by powder metallurgical techniques than by conventional ingot melting techniques.
The preparation of dispersion strengthened powders having improved properties by mechanical alloying techniques has been disclosed by U.S. Pat. No. 3,591,362 (Benjamin) and its progeny. Mechanically alloyed materials are characterized by fine grain structure which is stabilized by uniformly distributed dispersoid particles such as oxides and/or carbides.
Mechanical alloying, for the purposes of this specification, is a relatively dry, high energy milling process that produces composite powders with controlled extremely fine microstructures. The powders are produced in high energy attritors or ball mills. Typically the various elements (in powder form) and processing aids are charged into a mill. The balls present in the mill alternatively cause the powders to cold weld and fracture ultimately resulting in a very uniform powder distribution.
Aluminum, in particular, lends itself very well to lightweight parts fabrication--especially for aerospace applications. Aluminum, when alloyed with other constituents, is usually employed in situations where the maximum temperature does not exceed about 204.degree.-260.degree. C. (400.degree. F.-500.degree. F.). At higher temperatures, current aluminum alloys lose their strength. However, it is desired by industry to develop aluminum alloys that are capable of successfully operating up to about 482.degree. C. (900.degree. F.). Developmental work utilizing aluminum along with titanium, nickel, iron and chromium systems is proceeding in order to create new alloys capable of functioning at the higher temperature levels.
To date it has been extremely difficult to mechanically alloy aluminum alloys that contain elemental additions that are significantly harder than the aluminum matrix, i.e., aluminum with Ni, Fe, Cr, V, Ce, Zr, Zn and/or Ti. When directly processing these alloys at the desired composition, the aluminum powder cold welds around the harder alloy constituent forming composite powder particles of aluminum embedded with large, segregated, unalloyed elemental additions.